The objectives of this project are to: (1) develop an experimental model for subsequent studies on weaning from mechanical ventilation in animal and clinical populations, and (2) determine the effects of mechanical ventilation (MV) and weaning on biological time structures of hemodynamic and metabolic functions. The aims of this study are to (1) test the feasibility of using the pig as an animal model for studying the effects of mechanical ventilation and weaning on circadian rhythms in a critical care environment, (2) determine the effect of sedation on circadian rhythms, (3) test the stress effects of mechanical ventilation and weaning, (4) characterize the biologic time structure of hemodynamic functions over time, (5) characterize the biologic time structure of metabolic functions over time, and (6) identify critical markers(s) of biologic time structure during MV and weaning. Using chronobiologic methods and a repeated measures experimental design the investigators propose to investigate the effects of MV and weaning on hemodynamic and metabolic functions in a porcine critical care model. Four male, adult, conditioned pigs will be synchronized over 7 days to a 12-hour light:dark cycle. Continuous temperature will be monitored during synchronization to determine the time needed to entrain core body temperature (CBT). Then subjects will be sedated and instrumented with an endotracheal tube and catheters for continuous real-time monitoring of mean arterial pressure, hear rate, CBT, arterial pH, arterial oxygen tension, arterial carbon dioxide tension, and arterial oxyhemoglobin saturation and periodic measurements of serum catecholamines and cortisol. For 14 days, subjects will be cared for with a constant routine that simulates critical care. Data will be collected consecutively during 4 days of spontaneous breathing (baseline), 4 days of MV, 2 days of weaning and 4 days of spontaneous breathing (follow up). Mean arterial pressure, heart rate, core body temperature, arterial pH, PaO2 and SaO2 will be measured continuously throughout all study phases; serum catecholamines and cortisol will be measured two-hourly during the last 2 days of each study phase. Single cosinor analysis, autocorrelation, and periodogram analysis will be used to determine the profile and overlap of marker rhythms, ANOVA techniques will be used to compare baseline, mechanical ventilation, weaning and follow-up rhythms.